The present invention relates to angular position sensing transducers and in particular to a tape tension arm position sensing transducer preferably employing a Hall Effect device.
In the field of television, sophisticated broadcast videotape recorders utilize a tape tension arm in the magnetic tape path, to control the tape tension during the process of recording and reproducing magnetic history on the tape. In such situations, the position of the tension arm, that is, the rotation of the tension arm through preselected varying angles as it bears against the tape, is transduced to a voltage and supplied as one input to servo circuitry which controls the tension applied to the tape. Thus, sensing the position of the tension arm as defined in terms of voltage, provides a relative indication of the tension in the tape. Since tape tension is one parameter which is directly related to the accuracy of recording and reproducing magnetic history on tape, it is highly desirable that the position sensing transducer be very accurate, preferably supply a very linear voltage output relative to the angular position, and be temperature insensitive. In addition,it is highly desirable that the tension arm sensor be simple, readily manufactured and assembled, and mechanically and electrically reliable in its operating environment.
There are various types of tension arm position sensing transducers available at present for use, for example, in videotape recorders. Predominant among such devices is a photo potentiometer transducer which senses the relative angular position of the tension arm employing a light emitting diode (LED). The LED directs light through a mask having an opening of preselected shape, and thence into a photo-resistive potentiometer sensor. As the tension arm rotates, a correspondingly varied amount of light is passed by the mask and a photoresistor generates an output voltage which thus varies in proportion to the degree of rotation. Although widely used, such a photo potentiometer transducer necessitates the use of electric wires which extend from the sensing device to the sending device (LED) mounted on a printed wiring assembly. Since the LED is mounted on the rotatable shaft, the connecting wires cause drag which, in turn, causes corresponding errors in the output voltage which represents the angular position of the shaft. In addition, due to the characteristics of the photo potentiometer transducer, the generated output inherently is relatively non-linear, which also causes errors in the output voltage. The transducer further has the disadvantage of failing to provide consistently accurate readings under actual operating conditions in the tape recorder due to changing or inconsistent linearity characteristics of the photoresistor within the photo potentiometer which, in turn, deteriorates the accuracy of the associated servo circuit.
A second type of angular position sensing transducer employs a magneto-resistive transducing device, wherein rotation is translated into an output voltage via changes in resistance of the device caused by movement of a magnetic field. These transducers are very temperature sensitive and, due to their configuration, require sophisticated mounting within their respective supporting housing. The complexity in mounting the magneto restrictive transducer is exemplified by the difficulty in aligning the separate shaft, required in the device which performs the sensing, with the shaft whose rotation is being sensed. The transducer is not self-contained and therefore adds undesirable length to the assembly, whereby the added complexity in the supporting housing is reflected as additional costs in manufacture and assembly.
A third type of angular position sensing transducer utilizes a Hall Effect device in conjunction with a magnet for supplying a selected magnetic field. Such transducers provide selected advantages over the previous types of transducers mentioned above. However, the Hall Effect transducers presently available have configurations which inherently contain various disadvantages. For example, the transducers sense rotation of a member by utilizing a varying air gap to achieve varying flux densities. The Hall Effect device senses the varying flux density and generates a voltage proportional to the rotation of the member. However, the resulting voltage generated by this configuration may be relatively non-linear throughout the angle of rotation which is being measured due to the varying air gap configuration employed.
The present invention overcomes the disadvantages of the angular position sensing transducers of previous mention, while providing a Hall Effect transducer which further overcomes the disadvantages of Hall Effect transducers presently available. To this end, the invention provides a relatively simple configuration which readily is mounted as a self contained unit in, for example, an existing supporting housing of a videotape recorder tension arm structure, and which further provides a very linear output voltage over the entire angular rotation of the tape tension arm. In addition, the invention is temperature insensitive and, due to its simplicity, is very reliable. In turn, the transducer is readily manufactured and is easily assembled within, or added as a retrofit assembly to, for example, a videotape recorder.
To this end, in a preferred embodiment, the invention utilizes a Hall Effect device and magnet coupled to a tape tension arm shaft, in combination with an offset compensated, voltage regulated, amplifying electronic circuit. The shaft of a conventional tension arm is mounted within a housing, and a ring magnet, selectively polarized relative to the circular configuration thereof, is coaxially secured at the end of, or along the length of, the shaft for rotation therewith. The Hall Effect device is secured as by the housing in adjacent proximity to the rotatable ring magnet to provide a preselected constant gap between the Hall Effect device and the circumference of the ring magnet throughout the possible rotation of the shaft. The close and constant proximity of the Hall Effect device relative to the ring magnet optimizes the magnetic field seen by the Hall Effect device, and thus optimizes the linear voltage output. For example, in a preferred embodiment using a ring magnet polarized across the diameter to define poles spaced 180.degree. apart, the rotation may approach .+-.80 degrees from a zero or null position corresponding to a center position of the rotatable tension arm. To further optimize the output voltage linearity, the Hall Effect device preferably is located midway relative to the orientation of the magnetic poles of the magnet, that is, is located substantially at the magnetic null generated by the north-south poles, when the tension arm is in its center position. Thus voltage variations indicative of ring magnet rotation are caused by movement of the pole pair relative to the Hall Effect device, rather than by a flux density change such as caused by varying the air gap.
The Hall Effect device herein is supplied with a regulated voltage, and the output voltage therefrom is supplied to an offset operational amplifier which provides adjustment for setting the voltage to a selected nulled voltage, for example, to zero, when the tension arm is in its center position. A second operational amplifier is coupled to the offset operational amplifier and suitably amplifies the output signal for subsequent use in the associated servo circuit.